A well drilling head comprises a housing and a bearing assembly. The housing has a sidewall structure defining a central bore. The bearing assembly includes an outer barrel, an inner barrel, bearing units and a preload force exerting member. The outer barrel has a central bore and the inner barrel is removably seated within the housing central bore. The bearing units are coupled between the barrels for providing concentric alignment of the barrels and allowing rotation therebetween. A first one of the bearing units is configured for carrying loads exerted radially between the barrels and a second one of the bearing units is configured for carrying loads exerted longitudinally between the barrels. The preload force exerting member maintains force on the longitudinal load carrying bearing unit and said barrels thereby preloading the longitudinal load carrying bearing unit.